As treatment outcomes of primary or systemic cancer sites improve, the clinical importance of brain metastasis (BM) is growing. Twenty-four to 45 percent of all cancer patients develop BM, the majority from lung, breast or melanoma primary cancers, but few patients with BM live longer than a year, and BM constitutes 20% of annual cancer deaths. Ironically, recent advancement in chemotherapy has further increased the incidence of BM because most therapeutic agents cannot effectively penetrate the blood-brain barrier (BBB) and tumor cells find the brain as a sanctuary. Therefore, it is of paramount importance to have a deeper understanding of mechanisms that promote BM growth, which could be specifically leveraged to overcome current limitations in therapy. As opposed to the molecular mechanisms involving cancer cell?host interactions shared by multiple cancer types that result in organ specific metastasis, a highly distinct set of structural, anatomic, physiologic and molecular factors regulate metastasis to the brain. Astrocytes, the most common glial cell comprising ~ 50% of all human brain cells, are a well characterized perilesional component of BM and recent discoveries, including ours, provide compelling evidence that molecular crosstalk between astrocytes and cancer cells is integral to BM development. Although seminal findings indicate that interactions with astrocytes occur at both early and late stages of tumor colonization process, our understanding of the reciprocal astrocyte-cancer cell crosstalk is limited. In preliminary studies, we have employed our Cell-Cell Communication Explorer (CCCExplorer), a unique computational modeling tool, in identifying the novel PCDH7-EGFR, IL6-IL6R, and CCL5-CCR5 astrocyte-tumor crosstalk signaling in regulating BM. Based on these observations and in view of the secretory nature of glial cells, we propose here to test the hypothesis that crosstalk with astrocyte-derived secreted factors is critical for tumor cell colonization in the brain. Given that an even more complicated paracrine signaling network may dynamically evolve at different stages of BM development, and the interactions could provide both anti- and pro-metastatic stimuli to cancer cells, we will test our hypothesis through the following aims: 1) to assess therapeutic potential of the PCDH7- EGFR, IL6-IL6R and CCL5-CCR5 paracrine signaling in BM mouse models employing gain and loss of function and pharmacologic approaches in syngeneic mouse and human cancer xenografts; 2) to assess the astrocyte secreted proteins in modifying the function of BBB and microglia/macrophage in early BM; 3) to further characterize the temporally evolved astrocyte-BM cell crosstalks in a cancer type specific fashion. Our study is highly innovative in that (i) this study integrates knowledge and methods from both neuroscience and cancer to identify and characterize pro- and anti-metastatic astrocyte molecular mechanisms, their evolution during disease progression, and their manipulation in order to provide a valuable means of targeting astrocyte-cancer cell interactions. (ii) This study leverages powerful predictive modeling of cell-cell communications (CCCExplorer) to investigate and delineate the complex network of tumor-astrocyte interactions holistically in an unbiased manner. (iii) This study will address whether there is any specific therapeutic window as to which time point during BM might represent the most effective point of modulating and targeting the vicious astrocyte-tumor crosstalk. (iv) Given the strong response of astrocytes to BM during the course of brain colonization, the identification of secreted molecules may represent putative biomarkers of early diagnosis or response to therapy. (v) Data generated in this study would form an extraordinary repository for comparative analyses between different brain disorders to interrogate common and different aspects of astrocyte biology in different scenarios as well as to evaluate the potential new therapeutic strategies such as drug repurposing and combinations. The outcome of our study will provide a paradigm shift in current understanding of the pathology of BM, while achieving a significant impact on future treatments for this devastating disease.